In laboratory experiments, concentrations of 0.4, 0.8, and 1.2% (wt/vol) neem-seed extract inhibited feeding of adult and larva of Colorado potato beetle, Leptinotarsa decemlineata (Say), on treated potato foliage, Solanum tuberosum L. Adult mortality did not exceed 25% in any neem treatment; however, 73% of larvae were dead 72 h after feeding on foliage treated with 1.2% neem extract. Toxicity of neem extract to larvae was enhanced, and feeding of larvae and adults was inhibited to a greater extent by addition of the synergist piperonyl butoxide (PBO) at a ratio of 10:1 (PBO/azadirachtin). In field experiments, spray applications of neem extract significantly reduced numbers of L. decemlineata larvae and adults in treated plots on all sample dates. The effectiveness of neem extract against L. decemlineata was signifcantly improved on two dates by addition of PBO. The 1.2% neem + PBO treatment resulted in low tuber yield, suggesting possible phytotoxic effects.
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Zehnder et al. (1988) studied this question.